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Machinable Materials

What are Machinable Materials?

Knowledge of machinable materials and their properties is a crucial part of the expertise required in machine shops, machining factories, and for machinists. Machining different materials requires specific types of tools, methods, and cutting parameters to achieve the desired result that meets the requirements specified in the work drawings. ISO (International Organization for Standardization) has classified machinable materials into different material groups.

ISO Material Groups and Examples

ISO standards divide metallic materials into several classes based on their physical and chemical properties. Here are the five primary ISO material groups and some of their most common examples:

Steels (ISO P)

This group includes a wide range of materials from unalloyed to highly alloyed steels, including ferritic and martensitic stainless steels. Machinability varies depending on the material's hardness and other mechanical properties.

  • Structural steel (S235 & S355): Widely used structural steel that is easy to machine. Commonly used in construction applications.
  • Alloy steel (42CrMo4): An hardenable alloy steel, often requires special cutting parameters and tools.
  • Carbon steel (C45): A medium-strength carbon steel frequently used in component manufacturing, offering a good balance of machinability and wear resistance.

Stainless Steels (ISO M)

Stainless steels are materials containing at least 12% chromium. Other alloying elements may include nickel and molybdenum. Different structures, such as ferritic, martensitic, austenitic, and austenitic-ferritic (duplex), create a broad selection of materials. Common characteristics of all these materials include high heat generation, notch wear, and build-up edge formation.

  • Austenitic stainless steels (AISI 304 & AISI 316): Widely used in the food and pharmaceutical industries due to their corrosion resistance.
  • Duplex steels (2205): Combine the properties of austenitic and ferritic steels, providing high strength and corrosion resistance.
  • Sulfur-containing stainless steel (303): Contains sulfur to improve machinability.

Cast Iron (ISO K)

Unlike steel, cast iron produces short chips during machining. Gray cast irons (GCI) and malleable cast irons (MCI) are relatively easy to machine, whereas nodular cast irons (NCI), compacted graphite irons (CGI), and austempered ductile irons (ADI) are more challenging. All types of cast iron cause significant tool edge wear.

  • Gray cast iron (GG25): Widely used in machine parts and equipment. Besides good machinability and impact strength, it provides excellent damping properties.
  • Ductile cast iron (GGG40): Higher strength than gray cast iron, used in applications such as engine blocks and gears.

Non-Ferrous Metals (ISO N)

Softer metals, such as aluminum, copper, and brass. Aluminum with a silicon content of 13% is extremely abrasive. With sharp-edged tools, high cutting speeds are usually possible without significantly affecting tool life.

  • Aluminum alloys (Al6061 & Al7075): Commonly used in the aerospace and automotive industries for their light weight and good machinability.
  • Copper alloys (CuDn): Widely used in electrical components due to their excellent electrical conductivity.
  • Titanium alloys (Ti-6Al-4V): Require specific cutting parameters and machining methods, used for example in aerospace and medical industries.

Superalloys and Heat-Resistant Alloys (ISO S)

This group includes a large number of highly alloyed iron-, nickel-, cobalt-, and titanium-based materials. They are gummy, tend to form a build-up edge, work-harden during machining, and generate high heat loads. They are very similar to ISO M materials but are much more difficult to machine, resulting in shorter tool life.

  • Nickel alloys (Inconel 718 & Hastelloy C276): Used in the aerospace and chemical industries for their high resistance to heat and corrosion.
  • Cobalt alloys (Stellite 6B): Used in wear-resistant parts, such as valves and high-wear components, due to their high hardness and wear resistance.

Hardened Steels (ISO H)

Hardened steels: Include steels with a hardness of 45–65 HRC, as well as chilled cast iron (400–600 HB). Their hardness makes them difficult to machine, generating significant heat and causing considerable tool wear during machining.

  • Hardened Steels: This group comprises steels with a hardness range of 45–65 HRC and chilled cast irons around 400–600 HB. Their hardness makes them challenging to machine. Cutting operations generate heat and these materials are extremely abrasive on cutting tool edges.

Composites and Other Materials Not Defined in the ISO Standard (O)

Composites and non-metallic materials: Materials not defined in the ISO standard, such as thermoplastics, thermosets, glass fiber and carbon fiber reinforced plastics (GFRP and CFRP), carbon fiber composites, aramid fiber reinforced plastics, hard rubber, and technical graphite. The use of composites is becoming increasingly common in many industries, especially in aerospace.

Why Should You Know the Workpiece's ISO Material Group?

The machinability of workpiece materials significantly affects the profitability of machining operations. Incorrect selection of tools and cutting conditions during cost estimation can result in cost overruns and potential financial losses for your company.

By knowing the material group of the workpiece, the machinist can select the right tools and cutting conditions, improving efficiency and reducing costs. This is key to achieving profitability and customer satisfaction.

Summary

Understanding ISO material groups and the ability to select the appropriate machining methods and tools for different materials is a core competency in machine shops, machining facilities, and for machinists. By understanding the specific properties and machining requirements of various materials, it’s possible to ensure that manufactured components meet their technical and quality requirements. Proper material management and choosing the right machining strategies increase production efficiency and the quality of the final product, which is vital to maintaining competitiveness and ensuring customer satisfaction.